The present invention relates to rotary magnetic position sensors used to measure angular displacements.
The use of magnetoresistors (MRs) and Hall devices as position sensors is well known in the art. For example, a magnetically biased differential MR sensor may be used to sense angular position of a rotating toothed wheel, as for example exemplified by U.S. Pat. No. 5,754,042.
Position sensors with digital outputs provide discrete position information only whereas an analog position sensor can provide both position information and outputs that can be used to drive an electric motor or other similar electromechanical devices. Many of these devices are driven by sinusoidal excitations as a function of position. Consequently, an analog position sensor having an output that varies sinusoidally with position could be used to generate absolute angular positions as, for example, an electrical power steering system to measure the angle of rotation of the steering wheel, and/or reference signals to produce the desired sinusoidal phase drive currents and voltages to drive electric motors and other similar electromechanical devices.
Applications may further include throttle position sensors, pedal position sensors in drive-by-wire systems, body height position sensors for automotive suspension systems, a 3-phase sine generator for brushless motors, a sine/cosine resolver for servo motors, etc.
The operational principle of an angle encoder sensor is based upon the property of Hall plates and semiconductor magnetoresistors, collectively referred to herein as magnetosensitive devices, to sense only the normal component of the magnetic field passing through them. Consequently, if a constant and uniform magnetic field is rotated in the plane perpendicular to the surface of a magnetosensitive device, the output signal will vary as the cosine of the angle between the direction of the incident magnetic field lines and the line normal to the surface of the device. It is preferred in this regard, that the magnetosensitive device be linear in its response to change in direction of the incident magnetic field, such as that provided by Hall plates; however, magnetoresistors operating in their linear region can also be used. In addition, operation over any ambient temperature range may require temperature compensated magnetosensitive devices. Also, it should be noted that included by the term xe2x80x9cmagnetosensitive devicesxe2x80x9d are ferromagnetic magnetoresistors, including giant magnetoresistor (GMR) sensors, which can also be used although these are less preferred because their resistance versus magnetic flux density saturates at a relatively low level compared with Hall sensors that do not saturate.
Accordingly, what remains needed is a compact inexpensive contactless position sensor having a sinusoidally varying output suitable for specialized sensing schemes.
The present invention is an analog angle encoder, wherein rotation of a magnetic field relative to a magnetosensitive device provides a varying output of the magnetosensitive device that varies sinusoidally with the angle of relative rotation.
The analog angle encoder according to the present invention includes a non-magnetic stator which is preferably cup shaped, having an outer annulus, a bottom disk and a central post connected with the bottom disk and concentrically positioned relative to the outer annulus. Located within the central post is located at least one magnetosensitive device, the leads of which pass out from the cup. A rotor in the form of a ring captured between the central post and the outer annulus is provided, wherein the ring is rotatable relative to the stator.
At least one magnet is positioned within the rotor so as to provide a magnetic field incident upon the at least one magnetosensitive device. As the rotor rotates relative to the stator, the angle of the incident magnetic field changes relative to the magnetosensitive device, thereby causing the output from the device to vary sinusoidally with angular position.
While it is possible to use a single magnet, in order to provide a more uniform incident magnetic field it is preferred for a pair of opposed magnets to be used, located on diametrically opposite sides of the central post. In this regard further, it is preferred to use magnetic configurations other than simple six sided magnets in order to achieve a yet more uniform incident magnetic field, as for example by utilizing cylindrical magnets, arcuate (concave faced) magnets, flat ferromagnetic layer pole pieces, arcuate (concave faced) ferromagnetic layer pole pieces, and ferromagnetic return paths.
Accordingly, it is an object of the present invention to provide an analog angle encoder having improved signal response accuracy to sensed relative rotation.
This, and additional objects, features and benefits of the present invention will become apparent from the following specification.